With light emitting diodes, particularly ultraviolet light emitting diodes based on group III nitride semiconductor layers, increasing in complexity, efficiency, reliability, and lifetime, and thus the costs associated with the light emitting diodes, becomes increasingly important for customers.
Fabrication of high-quality aluminum gallium nitride (AlGaN) layers using epitaxy continues to be challenged by a lack of matched substrates. Threading dislocations that result from heteroepitaxy are responsible for leakage currents, trapping effects, and may adversely affect device reliability. Aluminum nitride (AlN) nucleation conditions have been determined to be important for reliability of the device when grown on silicon carbide (SiC) substrates. In particular, it was found that variation of the nucleation temperature, V/III ratio, and thickness have a dramatic effect on the balance between edge, screw and mixed character dislocation densities. Electrical and structural properties have been assessed on a material level by AFM and XRD and at the device level through DC and RF performance. The ratio between dislocation characteristics has been established primarily through comparison of symmetric and asymmetric XRD rocking curve widths.
There are situations where the presence of polarization fields in group III nitrides can dramatically change the performance of the device. For example, HEMT devices based on group III nitride heterostructure specifically employ polarization properties of the semiconductor layers for optimal performance.